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Paired t-test using Minitab

Introduction

The paired t-test (also known as the paired-samples t-test or dependent t-test) determines whether there is a statistically significant difference in the mean of a dependent variable between two related groups.

For example, you could use a paired t-test to determine whether there is a difference in students' test anxiety before and after undergoing a hypnotherapy programme designed to reduce stress (i.e., the dependent variable would be "test anxiety", and the two related groups would be the two different "time points"; that is, test anxiety "before" and "after" undergoing the hypnotherapy programme). Alternately, you could use a paired t-test to understand whether there is a difference in athletes' 100m sprint times when using a protein supplement compared to not using a supplement (i.e., the dependent variable would be "100m sprint time", and the two related groups would be the two different "conditions" participants were exposed to; that is, 100m sprint times when taking the protein supplement (condition A) compared 100m sprint times when not taking a supplement (condition B)).

In this guide, we show you how to carry out a paired t-test using Minitab, as well as interpret and report the results from this test. However, before we introduce you to this procedure, you need to understand the different assumptions that your data must meet in order for a paired t-test to give you a valid result. We discuss these assumptions next.

Minitab

Assumptions

The paired t-test has four "assumptions". You cannot test the first two of these assumptions with Minitab because they relate to your study design and choice of variables. However, you should check whether your study meets these two assumptions before moving on. If these assumptions are not met, there is likely to be a different statistical test that you can use instead. Assumptions #1 and #2 are explained below:

Assumptions #3 and #4 relate to the nature of your data and can be checked using Minitab. You have to check that your data meets these assumptions because if it does not, the results you get when running a paired t-test might not be valid. In fact, do not be surprised if your data violates one or both of these assumptions. This is not uncommon. However, there are possible solutions to correct such violations (e.g., transforming your data) such that you can still use a paired t-test. Assumptions #3 and #4 are explained below:

In practice, checking for assumptions #3 and #4 will probably take up most of your time when carrying out a paired t-test. However, it is not a difficult task, and Minitab provides all the tools you need to do this.

In the section, Test Procedure in Minitab, we illustrate the Minitab procedure required to perform a paired t-test assuming that no assumptions have been violated. First, we set out the example we use to explain the paired t-test procedure in Minitab.

Minitab

Example

A researcher wants to determine whether a hypnotherapy programme can help to reduce cigarette consumption amongst long-term smokers, defined as people that have been regular smokers for more than 10 years. Therefore, the dependent variable was "cigarette consumption", measured in terms of the average number of cigarettes smoked, and the independent variable was "time", which consisted of two related groups: "before" and "after" the hypnotherapy programme.

To carry out the experiment, the researcher recruited 20 long-term smokers. All of these 20 participants took part in the intervention, which was a 6 week hypnotherapy programme designed to help them quit smoking. The cigarette consumption of the participants was first recorded "before" the intervention (i.e., pre-intervention) and then for a second time "after" the intervention (i.e., post-intervention). This is typically known as a "pre-test post-test" study design.

A paired t-test was used to determine whether there was a statistically significant difference in cigarette consumption before and after the hypnotherapy programme.

Minitab

Setup in Minitab

In Minitab, we set up the two related groups as though they were two variables. Therefore, under column we entered the name of the first related group, Pre, as follows: . Then, under column we entered the name of the second related group, Post, as follows: . Finally, we entered the scores on the dependent variable for each of the two related groups (i.e., the cigarette consumption for each participant before the hypnotherapy programme in the Pre column and the cigarette consumption for the same participants after the hypnotherapy programme in the Post column). This is illustrated below:

Data setup for the paired t-test in Minitab

Published with written permission from Minitab Inc.

Note: If you do have all the data for your two related groups, as in our example above, but only the summarized data of the differences between your two related groups (i.e., the sample size, mean difference and standard deviation of the difference), Minitab can still run a paired t-test on your data. However, you will need to set up your data differently in order to do this.

Minitab

Test Procedure in Minitab

In this section, we show you how to analyse your data using a paired t-test in Minitab when the four assumptions set out in the Assumptions section have not been violated. Therefore, the three steps required to run a paired t-test in Minitab are shown below:

Minitab

Interpreting the paired t-test output in Minitab

The Minitab output for the paired t-test is shown below. This output provides useful descriptive statistics for the two related groups that were compared, including the sample size, mean, standard deviation and standard error of the mean, as well as actual results from the paired t-test.

Output for the dependent t-test in Minitab

Looking at the "Mean" column, you can see that cigarette consumption amongst participants was lower after the hypnotherapy programme (18.30 cigarettes in the Post row) compared with before the hypnotherapy programme (26.95 cigarettes in the Pre row), with a mean difference between the two time periods of -8.65 cigarettes (shown in the Difference row). Also, when comparing the two time periods across the Difference row, we can see that the standard deviation was 14.32 cigarettes (the "StDev" column) with a standard error of the mean of 3.20 cigarettes (the "SE Mean" column). Furthermore, the 95% CI for mean difference row shows a 95% confidence interval (95% CI) for the mean difference of -15.35 to -1.95 cigarettes.

In the final row of Minitab output, you are presented with an obtained t-value (T-Value) of -2.70 and the statistical significance (2-tailed p-value) of the paired t-test (P-Value), which is 0.014. As the p-value is less than 0.05 (i.e., p < .05), it can be concluded that there is a statistically significant difference between the two time points (Pre and Post). In other words, the difference between mean cigarette consumption before and after the hypnotherapy programme is not equal to zero. Minitab does not include the degrees of freedom, but these are simply the sample size (the "N" column) minus 1 (i.e., N – 1). Therefore, in our example the degrees of freedom are 20 – 1, which is 19.

Note: In addition to the paired t-test output above, you will also have to interpret (a) the boxplots you created in Minitab to check if there were any significant outliers and (b) the output Minitab produces for your Shapiro-Wilk test of normality to determine normality (see the Assumptions section earlier if you are unsure what these assumptions are). Remember that if your data failed either of these assumptions, the output that you get from the paired t-test procedure (i.e., the output we discussed above) might not be valid and you will have to take steps to deal with such violations (e.g., transforming your data using Minitab) or use a different statistical test.

Minitab

Reporting the output of the paired t-test

When you report the output of your paired t-test, it is good practice to include:

Based on the Minitab output above, we could report the results of this study as follows:

A paired-samples t-test was run on a sample of 20 long-term smokers to determine whether there was a statistically significant mean difference in cigarette consumption before and after a hypnotherapy programme. Participants' cigarette consumption was lower after the hypnotherapy programme (18.30 ± 10.31 cigarettes) than before the hypnotherapy programme (26.95 ± 7.74 cigarettes); a statistically significant mean decrease of 8.65 (95% CI, -15.35 to -1.95) cigarettes, t(19) = -2.70, p < .014.

To make your results easier for others to understand, you can also produce a bar chart with error bars (e.g., where the errors bars could be the standard deviation, standard error or 95% confidence intervals). Furthermore, you are increasingly expected to report an "effect size" in addition to your paired t-test results. Effect sizes are important because whilst the paired t-test tells you whether differences between group means are "real" (i.e., different in the population), it does not tell you the "size" of the difference. Minitab does not automatically produce effect sizes through the paired t-test procedure, but there is a separate procedure in Minitab to do so.

Portions of information contained in this publication/book are printed with permission of Minitab Inc. All such material remains the exclusive property and copyright of Minitab Inc. All rights reserved.

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